Firstly, extract your firmware using the Knowledge Base article - "How to: extract firmware of your player using the S1fwx tool" from
here.
Then you can modify it using the Knowledge Base instructions - "How to: Modify your firmware using s1res tool" from
here.
There are lots of themes available for download from the [Downloads] --> [Themes] section on this site.
Hope that helps.
If this answers your question, please put [solved] in the subject of your original post.